This invention relates generally to evaporators adapted to convert liquified gas into a gas, and more particularly to an evaporator chamber which is immersed in heated water, the chamber being protectively coated to inhibit corrosion and scaling without, however, degrading its heat transfer characteristics.
Though the invention is applicable to various forms of liquified gas, such as ammonia and sulphur dioxide, it will mainly be explained in connection with chlorine; for this gas, though toxic, is widely used in water purification, sewage treatment and in many industrial processes.
Chlorine evaporators of the type commercially available make use of a vapor chamber supported within a larger water chamber having an immersion heater therein. One such evaporator is manufactured by the Fischer & Porter Co. of Warminster, Pa., the device being described in their Instruction Bulletin for the Series 71V1000 Electrically Heated Evaporators.
In an evaporator of this type, water heated in the water chamber provides a uniform distribution of heat around the outer surface of the vapor chamber. As a result, liquid chlorine fed into the vapor chamber through an inlet pipe absorbs heat from the water chamber through the wall of the vapor chamber, causing the liquid chlorine to boil and converting it into a superheated gas which is discharged through an outlet pipe.
The vapor chamber which functions as a pressure vessel is generally made of carbon steel components that are welded together to define a leak-proof tank. There are two factors which are vitally important in the proper design of a vapor chamber of this type.
The first factor is the heat transfer characteristics of the vapor chamber, for it is essential that heat from the water in the water chamber be transferred without significant energy losses to the liquified gas. Carbon steel has excellent heat transfer characteristics, but because the chamber formed of this metal is immersed in water which normally contains dissolved oxygen, it is subject to fairly rapid corrosion and pitting caused by oxidation of the metal surface in contact with the water. Such corrosion degrades the heat transfer characteristics of the vapor chamber and may also affect its integrity. Moreover, the required high operating temperature accelerates the rate of corrosion.
The second factor is chlorine leakage. While highly beneficial as a hygienic agent, chlorine is hazardous as a free gas and serious injury may result to personnel in the vicinity of the evaporator by as little as 40 to 60 parts per million of chlorine gas in air inhaled for 30 minutes or more.
With a view to reducing the rate of corrosion, it is the current practice to provide cathodic protection. In the Series 71V1000 Evaporators, this is accomplished by suspending four sacrificial anode rods of magnesium in the water chamber surrounding the vapor chamber. These rods are the active elements of the protective circuit which operates on the electrochemical principle based on the flow of current between two dissimilar metals immersed in a conductive fluid. The current which flows from the more active magnesium anode to the less active cathodic carbon steel chamber surface is directed through a potentiometer and an ammeter to provide manual adjustment and visual indication of the magnitude of the protective circuit.
Cathodic protection is expensive both to install and to maintain, for the consumable anodes having a high replacement cost. Yet such protection is not fully effective, for the conditions which give rise to corrosion are many and varied, and even though a reduced electrochemical potential is created by the sacrificial anodes, an electrochemical potential can still exist to induce corrosive activity. Though the geometry of the anodes to the cathode is a controlling factor, it is impractical to place the anodes in the optimum position surrounding the vapor chamber, as a result of which some regions of the chamber are better protected than others.
Highly localized potentials can exist on the surface of the vapor chamber as a result of impurities and alloys present in the steel. Thus an intergranular electrochemical cell could be established between an iron and carbide grain structure. Since the water bath is not an ideal electrolyte, both of the above conditions are promoted. All of these conditions are highly variable, and some units therefore are more prone to corrosion than others despite carefully regulated cathodic protection.
Moreover, even if it works perfectly, the cathodic protection system affords no immunity whatever against scale build-up of a non-corrosive nature on the outer surface of the vapor chamber. This scale develops as a result of water hardness; that is, the proportion of calcium carbonate or calcium sulfate in a given sample of water. These constituents precipitate out and adhere to the steel wall to produce scale thereon which behaves as an effective thermal barrier, thereby degrading the heat transfer characteristics of the vapor chamber.
Existing ASME pressure-vessel codes dictate that to accommodate the design pressure of the vapor chamber, it must be constructed with a wall thickness of at least 0.305 inches. But because of the uncertain protection afforded by the cathodic system, it is the present practice to fabricate this chamber from steel of at least 1/2 inch thickness for increased strength and corrosion protection.
These extra heavy steel walls result in a unit that is extremely difficult to weld, as a consequence of which a number of passes on each joint is required. To ensure the absence of any flaws, all welds must be radiographically inspected, and if a flaw is detected it must be ground out and rewelded. Because of the thickness of the weld, it is difficult and sometimes impossible to repair a faulty weld. And as there are three welds per chamber, the probability for one faulty joint is fairly high. Should it not be possible to repair the weld, the unit must be scrapped.
Thus in the case of a vapor chamber designed as a vessel for a hazardous gas under pressure and immersed in heated water, existing measures to protect the chamber from corrosion and pitting and to prevent gas leakage are not only costly but they are not consistently effective. Hence it has heretofore been the practice to construct the chamber with a heavier gauge steel than is warranted by operating pressures. Apart from the additional expenses entailed by a thicker chamber wall, the welding problems created thereby further complicate manufacturing procedures, giving rise to significant scrap losses. And despite the steps heretofore taken to inhibit corrosion and to maintain safe operating conditions, the problem of scale build-up remains unsolved. As a consequence, with continuous use the heat transfer characteristics of the vapor chamber become impaired.